As shown in FIG. 1, Japanese Patent Laid-Open Publication No.302123/1995 discloses a pressure reducing valve α comprising a cylindrical casing 1, a valve support member 2 provided with a plate portion 2a, an annular portion 2b and a water passage 2c and fitting in an end of the casing 1, a valve seat 3 fitting on the valve support member 2, a cylindrical valve body 4 provided with an external flange 4b at its one end 4a, the external flange 4b fitting in the other end of the casing 1 to slide and the other end 4c being inserted in the annular portion 2b to slide, and a spring 6 disposed in a chamber 5 defined by the casing 1, the annular portion 2b, the external flange 4b and a cylindrical portion of the valve body 4 to force the external flange 4b away from the valve seat 3, wherein the pressure reducing valve is fitted in a city water pipe with the plate portion 2a directed toward the upstream side (hereinafter called the primary side). In the pressure reducing valve α, city water flows from the primary side to the downstream side (hereinafter called the secondary side) through the water passage 2c and an internal space of the valve body 4. When the water pressure in the secondary side city water pipe (hereinafter called the secondary pressure) increases due to the increase of the water pressure in the primary side city water pipe (hereinafter called the primary pressure), the valve body 4 moves to a position where the force applied to the external flange 4b by the secondary pressure balances with the force applied to the external flange 4b by the spring 6, the other end 4c of the valve body 4 approaches the valve seat 3, and the sectional area of the water passage 2c decreases. As a result, the secondary pressure is reduced to approach a predetermined level. When the primary pressure further increases, the other end 4c of the valve body 4 abuts the valve seat 3 to close the water passage 2c, thereby shutting off the water flow from the primary side to the secondary side. Thus, the secondary pressure is kept at the predetermined level.
As shown in FIG. 2, Japanese Patent Laid-Open Publication No. 320723/2000 discloses a pressure reducing valve β comprising a cylindrical casing 7, a valve support member 8 provided with a plate portion 8a, an annular portion 8b and a water passage 8c and fitting in an end of the casing 7, a valve seat 9 fitting in the valve support member 8, a cylindrical valve body 10 slightly spaced annularly from the other end of the casing 7 to slide at an external flange 10b provided at its portion near the one end 10a, and inserted in the annular portion 8b to slide at the other end 10c, and a spring 12 disposed in a chamber 11 defined by the casing 7, the annular portion 8b, the external flange 10b and a cylindrical portion of the valve body 10 to force the external flange 10b away from the valve seat 9, wherein the pressure reducing valve is fitted in a city water pipe with the plate portion 8a directed toward the primary side. In the pressure reducing valve β, city water flows from the primary side to secondary side through the water passage 8c and an internal space of the valve body 10. When the secondary pressure increases due to the increase of the primary pressure, the valve body 10 moves to a position where the force applied to the external flange 10b by the secondary pressure balances with the force applied to the external flange 10b by the spring 12, the other end 10c of the valve body 10 approaches the valve seat 9, and the sectional area of the water passage 8c decreases. As a result, the secondary pressure is reduced to approach a predetermined level. When the primary pressure further increases, the other end 10c of the valve body 10 abuts the valve seat 9 to close the water passage 8c, thereby shutting off the water flow from the primary side to the secondary side. Thus, the secondary pressure is kept at the predetermined level. The secondary pressure is applied to the external flange 10b through the narrow annular space between the one end 10a of the valve body 10 and the casing 7. Therefore, even if the secondary pressure increases quickly due to the closure of a closing valve disposed secondary side, the force applied to the external flange 10b by the secondary pressure does not increase quickly because of the pressure loss caused by the narrow annular space, and the water passage 8c is not closed quickly. Therefore, no water hammer is caused at the primary side due to the closure of a closing valve disposed at the secondary side.